Does all food metabolize into glucose?

It seems to me that I read somewhere that all food eventually is metabolized into glucose (or maybe glycogen?) I realize the question is almost certainly high school science material, but I am having a difficult time coming up with information, because when I search “metabolism” I get 4,454,354,754 hits for diet pills! I knew that some of the extremely intelligent folks here on the message boards would be able to answer the question. Can anyone confirm or refute this and perhaps steer me to a source?

Plants and some bacteria can convert fatty acids to carbohydrates via the Glyoxylate cycle. We mere mammals cannot.

I am forever in your debt, sir (or madame, Squink being gender neutral). That was just the information I was looking for.

yes fatty acids are converted directly to acetyl-CoA (and similar), which is the key feedstock for the Krebbs energy cycle. Glucose is also broken down to acetyl-CoA. A comprehensive summary can be found here

Whenever I hear “all”, “always”, “never”, etc., I tend to shy away because there’s always(!) an exception. But for this question, it’s clearly not “all”.

Here’s one example: When you’re starving, some of the fat you ate some time ago is metabolized by the liver into what’s called a ketone. The ketones so produced are used as an energy source by the brain and by the muscle tissue.

In other words, dietary fat can fuel the brain and muscle without ever being metabolized to, or from, glucose.

[After I posted, I see that scm1001 has said something similar (note that acetyl-CoA is metabolically “in between” the fat and the ketone.]

Not entirely true; there are a small number of fatty acids into glucose via gluconeogenesis in the anaerobic respiration cycle. This is highly inefficient and produces waste products that then have to be filtered and rejected, but technically humans can reduce some fatty acids into glucose, though not enough to live on long term; we need readily available sources of glucose or simple sugars that can be readily converted to glucose in the form of dietary carbohydrates.

However, to answer the o.p.'s question, no, not all food is metabolized into glucose. You need various fatty acids to build natural steroids and cellular membranes (among other things) and proteins containing the essential amino acids that the human metabolism can’t synthesize, as well as non-organic compounds and elements which are used for neurochemical processes and regulation of membrane permeability. Glucose the fuel that powers all this, but it can’t be used to build anything.


Not just when you’re starving. On the Atkins diet, a state of ketosis is deliberately induced to put the body on a fat burning metabolism. You need to have some glucose to run the brain, but around a third of the protein consumed metabolizes to glucose, and provides sufficient glucose for cerebral needs. Atkins dieters usually stay below 50 or so grams of glucose per day, except during the two week induction when the glucose consumption stays down around 20 grams per day.

Alcohol (ethanol) does not metabolize into glucose. It becomes acetic acid and finally CO2 and water.

I dunno that I’d call it a “food”, though.

Stranger, the breadth and depth of your fund of knowledge is very impressive.

It’s better to say that most food which is used for energy usually gets converted to acetyl CoA (not glucose, b/c fat is not converted to glucose).

Here’s a simplified way to look at it:

Most of the energy the body uses is produced in the Krebs cycle–a series of reactions which produce the actual energy-yielding molecules.

There is a separate discussion around storage, transport and waste, but I think what you are after is the conversation around around stuff we eat that is burned as fuel–i.e. is converted to molecules used for energy.

The major catabolic pathways which prepare food to be used as energy all produce acetyl CoA, which can be considered a starting point for the main part of the TCA (Krebs) cycle.

Food used for energy can be conveniently grouped into carbohydrates, proteins and fats.

Carbohydrates and proteins are broken down into glucose, which is in turn converted to pyruvate which is converted to acetyl CoA which then goes down the rest of the TCA pathways.

In humans, fats do not have glucose as an intermediate molecule between the fat molecules being burned for fuel and acetyl CoA.

Generally speaking, different tissues “prefer” different substrates and not all tissues are able to perform all of the metabolic pathways. The brain, for instance, likes glucose.

It’s a bit off topic, but most of the time these conversations come up relating to dieting. Briefly, the dilemma is that the body stores fat as if it were a retirement account and for the most part uses carbohydrates and proteins for energy. The “trick” to getting rid of excess fat stores is to switch over to burning more fat.

An equally important concept is that the body has no trouble using carbohydrates and protein to make fat. You might say that most (certainly not all, but most) excess glucose (carbohydrates) you eat ends up as fat.

I am simply amazed.

Actually, the original question arose in an EMS class dealing with diabetic emergencies. A classmate said that proteins and fat were not metabolized into glucose. I think he was trying to convince us that if everyone was on the Atkins diet, there would be no diabetic emergencies. I disagreed. I said that I believed that most of what we eat eventually was converted into glucose, but at different rates and through different processes. He again disagreed (actually, he disdainfully said that I needed to take a nutrition class). I thought it best to distrust my dim memories of basic biology and come to the message boards, where actual (as opposed to those who just think they are) smart people could help me.

Isnt the majority of the ATP that the body uses produced via chemiosmosis in the mitochondrial membranes (following the krebs cycle, which doesnt produce much actual ATP)?

Proteins can be broken down into glucose? Its my understanding that proteins and fats can be broken down and fed into either glycolysis or the kreb’s cycle…but they are broken down glucose derivatives (pyruvate, etc.), not glucose itself. So – correct me if i’m wrong – that would make carbohydrates the only molecules that can be broken down into actual glucose.

Correct, but the oxidative phosphorilation could not happen without the Kreb’s cycle feeding it high energy electrons from NADH and FADH.

Well, your body will make glucose from fat and protein if it needs glucose. It can get energy from fat and protein directly, but it needs some glucose around for things like the brain and anaerobic energy. Fats and proteins feed directly into Kreb’s and Kreb’s doesn’t run in the absence of O2. Glycolysis is the only source of anaerobic energy in eukaryotes that I know of.

Right, we’re on the same page. And some NADH from glycolysis as well.

This is what you actually work with Stranger on a Train, isn’t it. Molecular biology?

Not me. I just try to make rockets not fall over. Molecular biology and evolutionary zoology are just dilettante enthusiasms for me.


Are you saying you’re a rocket scientist?! If so, you sure are the stereotypical smart guy. Anyway, you are one of very few posters who I click on to look at all his recent posts. Keep up the good work. :slight_smile: